GEOLOGICAL NOTES ON THE FALCHAN KANGRI (Broad Peak 8047 m) IN KARAKORAM

ARDITO DESIO

THE FALCHAN Kangri¹—the third highest peak of the Karakoram—- stands up between the Godwin Austen glacier, to the west, and the North Gasherbrum glacier to the east. Toward the north it ends in the upper valley of the Godwin Austen glacier, just in front of the southeast face of K².

From the west the summit of the mountain has a dome-shaped form truncated towards the lower Godwin Austen glacier by steep walls. The highest ridge is divided into three main peaks of which the southern is the highest.

The east side of Falchan Kangri descends steeply to the North Gasherbrum glacier (Shaksgam basin). I take as southern limit of the mountain the Falchan-la, the lowest saddle of the ridge joining the Falchan Kangri to the Gasherbrum. Northwards the limit of the Falchan Kangri is taken on a saddle at the; head of the Kharut glacier, a tributary of the Godwin Austen glacier (Fig. 1).

The approach of the intricate geological structure of .this massif is facilitated by the different colours of some rock formations and by the nature of the morainic debris conveyed by the glaciers flowing down from the mountain, Thanks to the relatively short lengths of these glaciers, it is, in fact, a fairly easy task to identify the origin of the rocks of the erratics. Further data was provided by some rock fragments collected m situ by Marcus Schmuck, leader of the 1957 Austrian expedition to Falchan Kangri.

About the stratigraphy, I recall that the same sedimentary rocks (conglomerate and sandstone) of the Savoia Glacier valley and the Khalkhal ridge² constitute a large portion of the western side of Falchan Kangri, outcropping between the ridge descending from Falchan I North and the southern glacier of Falchan Kangri (Fig. 1)

 

The basement of the sedimentary formations is a metamorphie complex (Falchan gneiss) mainly composed of quartz-feldspar gneiss and biotite paraschists which are technically separated from the overlying sedimentary sequence.

Light grey-green quartz-feldspar gneiss probably form a considerable portion of the top of Falchan II North. From a distance it shows a light green colour, differing from the colour of the other gneisses. The south wall of the NNW Falchan glacier valley is made up of fine-grained biotite gneiss with plagioclase porphyroblasts, similar to the 'K² gneiss' in which occur huge fragments of grey limestone forming a sort of cyclopic breccia. Above this breccia a shear zone is marked by another breccia composed by fragments of the basement gneiss and of the calcareous rocks of the overthrusted beds. These beds are grey strongly laminated calc-schist and limestone, crossed by numerous calcite veins, belonging to the Savoia formation. Toward the south thin beds of yellowish and grey limestone, intensely folded, outcrop.

Further south, in the long spur flanking the Godwin Austen glacier occurs the contact of the limestones of the Savoia formation and the complex of sericitic sandstone with light green phillite of the Kalkhal formation. This contact is conformable, whilst two faults, parallel to the bedding planes, intersect the limestone sequence.

The southern end of the spur flanking the SW Falchan glacier is entirely composed of slate and green sandstone, whilst on the opposite side of the glacier outcrops a sequence of light-coloured marbles, followed by black slates, similar to those of the Marble Peak on the other side of the Godwin Austen glacier.

Observations made from a distance of the highest slopes of Falchan Kangri with the help of samples collected from the moraines of the glaciers flowing from these slopes yielded data on the geological structure of the massif. The fig. 2 can substitute a detailed description of this part of the mountain.

I want only to add here the data supplied by the samples of M. Schmuck:

'I. Broad Peak—-7-6-1957, 5800 m. Collected near Camp 1 (2 small samples),

1. multicoloured conglomerate with liver-red arenaceous schistose cement, with very flattened small pebbles of white and reddish waxy subcrystalline limestone;
2. variegated white and red waxy subcrystalline limestone, crossed by many calcite veinlets.

'II. Broad Peak—collected north of the Camp 1, 5800 m. The Broad Peak on its western flank is crossed by black veins of this rock, several metres wide, which are inclined upwards from left to right.' Blackish "baltorite" rich in black mica.

'III. Broad Peak—7-6-1957, 8030-8047 m. (5 small samples). The rocks belong to two types, i.e.:

1. grey sandstone very similar to those of the Khalkhal formation;
2. whitish arenaceous rock, deeply weathered, composed of grains of feldspar and quartz and dark minerals perhaps derived from biotite.'

The most interesting of Schmuck's samples are the last ones, because they were collected between the northern pre-summit and the major summit of the Falchan Kangri. Both the rock specimens belong to the Khalkhal formation.

Go now to the southern side of our mountain. The floating moraines of the South Falchan glacier collect material from three main ice- flows coming from NNE, NE, and E. Only the first two descend from the Falchan massif, the third from the neve-fields common to Falchan Kangri and Gasherbrum ridge. The right-hand moraine is mainly composed of white and grey marble and limestone, less frequently black and rose-coloured waxy limestone, flesh-red, green, and yellow calcareous breccias. Ked conglomerate blocks occasionally occur, ' as well as fragments of gneiss, fine-grained diorite, and sgricitic schists. Toward the central moraine diorite blocks become prevsfent along the middle of the glacier. Proceeding toward the left-hand side of the glacier, grey and yellowish limestone associated with diorite and baltorite prevail. The left-hand side moraine is essentially composed of black biotite schists and marbles similar to those of the Marble Peak. These rocks outcrop in situ at the head of the valley and on the ridge of Falchan-la. Among the samples of particular interest are the dark grey biogenic limestones with organic remains of fora- minifera, including Neosclnvagerina, belonging to the Shaksgam formation of Permian age. Some limestones have been metamorphosed to calciphyre, marble and hornfels by intrusion of diorite, which produced a contact metamorphic aureole.

The distribution of diorite outcrops on the SW side of the mountain can be seen in Fig. 3. The diorite underlies the ridge of the Falchan-tso, outcropping here and there as satellite masses in the northern branches of the South Falchan glacier valley. But the main bulk is located further south in the Gasherbrum ridge. We have no available data on the geology of the eastern wall of the Falchan Kangri overhanging the catchment area of the North Gasherbrum glacier. I had the occasion to visit the lower tongue of this glacier and to examine attentively the morainic pebbles, but most of them came from other sides of the glacier valley.³

 

About the structure of the Falchan Kangri (Fig. 4), two principal dislocation systems are to be distinguished. One system is more or less parallel to the bedding planes and is represented by overthrust surfaces whose importance is difficult to determine. One of these surfaces separates the sedimentary formations from the metamorphic ones, and a layer of Permian (?) limestone is included between the metamorphic rocks and the Khalkhal arenaceous formation. Probably the whole upper portion of the massif, composed of the Khalkhal sandstone, is overthrust on the Permian limestone, and is thus affected by a system of numerous fractures and faults.

The Savoia limestone, together with a strip of Khalkhal sandstone, constitutes the western side of Falchan Kangri and rests on the inclined blanket of the older metamorphic and sedimentary formations. It is not clear whether they are part of the same overthrust mass, or whether they constitute an independent overthrust body. The presence of the ice blanket and the complicated tectonics make a precise analysis practically impossible. The structure is probably that of an anticlinal overthrust mass with its axis running high up on the western side of the mountain. Two younger east-west faults cross all the mentioned formations and the tectonic planes. They are both reverse faults, the northern one with a relatively minor throw, and the southern one with a throw of some hundreds of metres.

Much of the southern and central part of the Falchan Kangri is occupied by the diorite body, which cross-cuts the sedimentary formations and is therefore younger than the Khalkhal sandstone occurring in the highest parts of the massif. The major dislocations described above do not affect the diorite body, though to the south the contact of the igneous rock with the Gasherbrum limestone is probably due to a more recent fault.

A system of numerous, less important faults crosses the Falchan Kangri with an east-west or east-northeast trend. These faults of the main crest of the massif proceed downwards along the southern slope of the mountain and cross the long western spur dividing the South Falchan glacier from the Godwin Austen glacier. As previously stated, this spur is composed of Savoia and Khalkhal formations of Cretaceous age, so that the faults must be more recent than that age. But some of the small faults cross also the diorite body which is more recent than the above mentioned formation. The age of this fault system must therefore be late Tertiary.

 

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